This invention relates generally to dc/ac inverter apparatus and, more particularly, to dc/ac inverter apparatus configured to selectively drive either a three-phase motor or a single-phase motor. The invention also relates generally to variable-voltage power systems incorporating a photovoltaic array and the like, for powering loads such as the inverter apparatus and, more particularly, to circuits and techniques for preventing an excessively high input voltage from damaging the load.
Inverter apparatus of this particular kind are commonly used to power ac motors using dc power derived from photovoltaic arrays and other variable-voltage power sources. Both three-phase motors and single-phase motors have been driven using inverters of this kind. Three-phase motors typically include three input terminals and three inductive windings arranged either in a Y configuration or a delta configuration. Single-phase motors, on the other hand, typically include two input terminals and two windings, with a main winding and a supplemental winding. The supplemental winding typically has the same inductance as, but a higher resistance than, the main winding, and it is commonly used with a series capacitor for starting.
Inverter apparatus for driving three-phase motors provide three ac output voltage signals that are phased at 120.degree. relative to each other so as to efficiently drive the motor. This is accomplished using three pairs of switches, typically high-speed power transistors, with each pair being connected in series between an input terminal carrying the dc input voltage and a reference terminal carrying a negative reference voltage, or ground. Reverse-biased diodes are connected across each transistor, for use when switching inductive loads, such as motors. The nodes between the three pairs of switches, or poles, constitute the inverter's three output terminals. Generally, one transistor or the other of each pole is switched ON at any one time, and the duty cycles of the switching are controlled such that each pole simulates an ac voltage having the desired frequency and phase angle. The three-phase motor thereby is driven at a speed proportional to the ac voltage frequency.
Inverter apparatus for driving single-phase ac motors of the kind described above typically provide a single ac voltage signal for coupling through the motor's main winding and via a capacitor through the motor's supplemental winding. A second terminal of the motor couples the node between the two windings to a negative voltage reference, or ground. Inverters of this kind typically have included two pairs of switches or poles, again typically high-speed power transistors, and a controller switches ON just one transistor of each pole at a time. The duty cycle of the switching is controlled so as to simulate an ac voltage waveform having the desired frequency.
The inverter apparatus described briefly above have functioned satisfactorily to drive the three-phase or single-phase motors for which they have been configured. Sometimes, however, it is desirable to provide for the selective use of either a three-phase motor or a single-phase motor. In the past, this generally has required two separate inverters, one configured for three-phase motors and the other for single-phase motors. This adds significantly to the apparatus' expense and complexity. There is a need for an inverter apparatus that can conveniently be used to drive either a three-phase motor or a single-phase motor without requiring any substantial hardware reconfiguration. The present invention fulfills this need.
Inverter apparatus of the general kind described above, as well as other electrical loads, are vulnerable to damage from the application of excessive input voltages when they are powered by a variable dc power source such as a photovoltaic, or solar, array. Such power sources are considered to be "soft," meaning that their voltage levels can vary over a wide range, depending on several factors, including current loading and temperature. FIG. 1 depicts a voltage versus current relationship for one typical photovoltaic array. It will be noted that the array's voltage level drops monotonically with increasing current draw and that one particular combination of voltage and current provide maximum power output. Photovoltaic power systems typically are controlled to operate at or near this peak power point. It will be noted that the open-circuit voltage level is substantially greater than the voltage level at the peak power point. In addition, the voltage level varies significantly with variations in the array's temperature, and the current level varies substantially with variations in the incident sunlight, or insolation.
Because the photovoltaic array's open-circuit voltage can be substantially higher than the array's voltage at the peak power point, particularly at cold temperatures, appropriate steps must be taken to prevent this high voltage from damaging the load, e.g., an inverter apparatus. Typically, this is achieved by configuring the load to withstand the application of such a voltage. This can dramatically increase the load's cost and complexity. A need therefore exists for a less costly and less complex means for preventing the application of such high voltages to a load such as an inverter apparatus. The present invention fulfills this need.